A variety of peptides have been reported having various functions, such as hypotensive effect, anti-bacterial activity, calcium solubilizing effect, and immunomodulating effect, and these peptides are in use in food and medicine.
For example, hypotensive peptides have been proposed, many of which have angiotensin converting enzyme (abbreviated as ACE hereinbelow) inhibitory activity. ACE converts a precursor, angiotensin I, to angiotensin II having vasoconstrictive activity in living organism, to thereby raise the blood pressure. Thus peptides having ACE inhibitory activity are expected to exhibit hypotensive effect by inhibiting ACE to suppress production of angiotensin II in living organism. In the development of hypotensive peptides, researches are usually directed first to peptides having ACE inhibitory activity, so that hypotensive effect of the proposed peptides are mainly indicated by ACE inhibitory effect as reference index. A great number of agents having hypotensive effect evaluated by the ACE inhibitory activity as reference index, have been proposed to date, and are in use for prevention and treatment of hypertension.
In production of functional peptides, such as peptides having hypotensive effect evaluated by the ACE inhibitory effect, in particular, in production of functional peptides by digestion of food protein with enzymes, complicated steps are usually required after the enzymatic digestion, such as concentration, purification, and isolation of the digested products, for improving expression of the functional effects.
As peptides that are absorbed through the alimentary canal into blood to express their functions in living organism, in vivo indigestible peptides are expected to be advantageous that have high absorbability and digestion resistance against various digestive enzymes in living organism. However, it is not known in detail which peptides contribute to enhancement of the in vivo digestion resistance. Thus development of enzymatic digestion products for food or medicine and methods for producing such products are desired, which products have high in vivo digestion resistance, and can effectively express their desired functions without undergoing complicated processes such as concentration, purification, or isolation after the enzymatic digestion.
For example, Patent Publication 1 discloses a method for producing a low molecular weight peptide mixture mainly composed of dipeptides and tripeptides, having an average chain length of not longer than 3, and having excellent intestinal absorbability. The peptide mixture is prepared from soybean protein by simultaneous or sequential action of two or more enzymes having endoprotease activity but substantially no exoprotease activity, with not higher than 5% free amino acids being generated. Patent Publication 2 discloses functional food utilizing soybean protein digestion products and method for producing the same. The digestion products contain as active ingredients a dipeptide and tripeptides consisting of Ala-Tyr, Gly-Tyr-Tyr, Ala-Asp-Phe, and Ser-Asp-Phe, prepared by digesting heat denatured soybean protein with enzymes such as proteases derived from Aspergillus oryzae. The digestion products preferably have an average peptide chain length of 2 to 4, and contain 20 to 30 wt % free amino acids.
However, these enzymatic digestion products of soybean protein have components that are quite different from those of enzymatic digestion products of animal milk casein. Thus the above patent publications suggest nothing about methods for preparing, from animal milk casein as a staring material, casein digestion products and casein hydrolysate that have a high concentration of active ingredients and excellent absorbability in living organism, and may be used without necessarily undergoing complicated processes such as concentration, purification, and isolation.
On the other hand, Patent Publications 3 and 4 propose methods for preparing peptides having various functions by digesting animal milk casein with enzymes such as proteases and peptidases, and particular functional peptides obtained by the methods.
The enzymatic digestion products disclosed in these publications are, however, for obtaining particular peptides as active ingredients. Thus there is no teaching in these publications as to hydrolysis of animal milk casein to have an average chain length of not longer than 2.1, specific process of the hydrolysis, and effectiveness of the casein hydrolysate having the particular average chain length.
Patent Publication 1: JP-5-252979-A
Patent Publication 2: JP-2003-210138-A
Patent Publication 3: JP-6-128287-A
Patent Publication 4: JP-2001-136995-A